Polymerization of Ethylene Catalyzed by New Titanium and Zirconium Complexes with Fluorinated β-Imineenolato Ligands
Sze-Man Yu and Stefan Mecking*
Chair of Chemical Materials Science, Department of Chemistry, University of Konstanz, Germany.
INTRODUCTION
Living olefin polymerization is of interest, for example, for the preparation of well-defined polyolefins with narrow molecular weight distribution, and for the preparation of block copolymers. A limited number of suitable polymerization catalysts is known to date.1,2 Fujita et al. have reported bis(phenoxyimine)titanium dichloro complexes with ortho-fluorinated N-aryl moieties to be versatile precursors to living olefin polymerization catalysts.3 The fluorine substituents are considered to suppress chain transfers by interaction with the β- hydrogen atoms of a growing polymer chain. In similar complexes with other N,O-chelating ligands, however, ortho-fluorinated substitution was found not to result in living polymerizations.4 This raises the question whether retardation of chain transfer by β-H-fluorine interactions is restricted in fact to phenoxyimine complexes, or represents a more general principle.
EXPERIMENTAL
Materials. All manipulations of air- and/or water-sensitive compounds were carried out under an inert atmosphere using standard glovebox or Schlenk techniques. All glassware was flame-dried under vacuum before use. Toluene and benzene-d were distilled from sodium. Dichloromethane-d was distilled from CaH . [Cl Zr(CH Ph) *THF] was supplied by MCAT (Konstanz, Germany).
6
2 2
2 2 2
[Cl Ti(NMe ) ] was prepared by comproportionation from TiCl and [Ti(NMe ) ], which were purchased from Aldrich. 1,1,1-trifluoro-2,4- pentanedione and 2,6-difluoroaniline (98% purity) were obtained from ABCR. Methylalumoxane (MAO) was purchased from Crompton as 10 wt-%-solution in toluene, which was concentrated in vacuo, and stored as a solid white powder.
2 2 2 4
2 4
DSC data was recorded at 10 K min-1, from 2nd heats. Gel permeation chromatography (GPC) was carried out in 1,2,4- trichlorobenzene at 160 °C on a Polymer Laboratories 220 instrument equipped with Mixed Bed PL-columns. Data reported were referenced to linear polyethylene standards.
4-(2,6-difluorophenylamino)-1,1,1,-trifluoropent-3-en-2-one (1). To a stirred solution of 1,1,1-trifluoro-2,4-pentanedione (3.0 g, 19.5 mmol) in dry toluene were added 2,6-difluoroaniline (3.0 g, 23.4 mmol) and p-toluenesulfonic acid as a catalyst. The mixture was refluxed for 24 h in the presence of molecular sieves. After evaporation of the solvent, the crude product was recrystallized from ethanol to afford 1 in 75% yield.
1H NMR (400 MHz, C6D6, 25°C): δ/ppm= 12.00 (br, 1H, NH), 6.44 (m, 3JHH= 8 Hz, 1H, H para to NH), 6.30 (t, 3JHH= 8 Hz, 2H, H meta to NH), 5.40 (s, 1H, vinylic HC=C), 1.23 (s, 3H, CH3) . 13C NMR (100 MHz, C6D6, 25°C): δ/ppm= 178.23 (q, 2JCF= 33 Hz, CF3CO), 168.94 (C- CH3), 157.98 (dd, 1JCF= 250 Hz and 3JCF= 4 Hz, ortho C), 128.90 (C para to NH), 114.94 (t, 2JCF= 16 Hz, ipso C), 118.15 (q, 1JCF= 287 Hz, CF3), 111.82 (dd, 2JCF= 19 Hz and 4JCF= 5 Hz, meta C), 91.85 (vinylic C), 18.78 (CH3). 19F NMR (376 MHz, C6D6, 25°C): δ/ppm= -80.54 (s, 3F, CF3CO) and -122.46 (t, 3JFH= 8 Hz, 2F, aromatic F). Anal. calcd.
(%) for C11H8F5NO: C, 49.82; H, 3.04; N, 5.28; Found: C, 50.01; H, 3.01; N, 5.37. MS (m/z, %): 265 (31.2%, M+).
Dichloro-bis[к2-N,O-4-(2,6-Difluorophenylimino)-1,1,1-
trifluoropent-2-en-2-olato]zirconium(IV) (2). A solution of 1 (0.1 mmol) in toluene was added dropwise to a toluenic solution of [Cl Zr(CH Ph) *THF]2 2 2 (0.05 mmol) at - 30°C. A colour change was observed immediately. After evaporation of solvent in vacuo, the analytical pure complex was obtained.
1H NMR (400 MHz, CD2Cl2, 25°C): δ/ppm= 7.36-7.27 (m, 1H, para H), 7.05-6.97 (m, 2H, meta H), 6.03 (s, 1H, vinylic CH), 2.01 (s, 3H,
CH3). 13C NMR (100 MHz, CD2Cl2, 25°C): δ/ppm= 180.93 (imine C), 156.87 (q, 2JCF= 36 Hz, CF3CO), 155.56 (d, 1JCF= 253 Hz, ortho C), 129.19 (para C), 123.66 (ipso C), 118.82 (q, 1JCF= 277 Hz, CF3), 112.85 (t, 2JCF= 19 Hz, meta C), 105.23 (vinylic C), 25.42 (CH3). 19F NMR (376 MHz, CD2Cl2, 25°C): δ/ppm= -74.24 (s, 3F, CF3CO), - 118.09 (s, 2F, aryl F). Anal. calcd. (%) for C22H14Cl2F10N2O2Zr: C, 38.27; H, 2.04; N, 4.06; Found: C, 38.37; H, 2.07; N, 4.07.
Crystal data for 2: C22H14Cl2F10N2O2Zr, M= 690.47 g mol-1, monoclinic, C2/c, a= 10.313(9), b= 13.317(1), c= 19.161(2), α= 90.00°, β= 95.468(9)°, γ= 90.00°, V= 2619.8(1) Å3, T= 100 K, Z= 4, µ(Mo-K α)=
0.719 mm-1, Dc= 1.751 g*cm-3, 17115 reflections measured, 2789 independent (Rint= 0.0679), F2 refinement, R1= 0.0272. wR2= 0.0598, 2428 independent observed reflections [I> 2σ(I)], 205 parameters.
Dichloro-bis[к2-N,O-4-(2,6-Difluorophenylimino)-1,1,1-
trifluoropent-2-en-2-olato]titanium(IV) (3). A solution of 1 (0.1 mmol) in toluene was added dropwise to a toluenic solution of [Ti(NMe)2Cl2] (0.05 mmol) at –30°C. A colour change was observed immediately.
After filtration, 1 eq. of BCl3 in toluene was added to the intermediate at -30°C. The reaction mixture was warmed up to room temperature.
Within 30 min, the solution turned from blackish to red. Precipitation is observed. The mixture was stirred for 3 hours. Evaporation of the solvent afforded 3.
1H NMR (400 MHz, C6D6, 25°C): δ/ppm= 6.50-6.30 (m, 3H, aryl H), 5.28 (s, 1H, vinylic CH), 0.95 (s, 3H, CH3). 19F NMR (376 MHz, C6D6, 25°C): δ/ppm= -74.51 (s, 3F, CF3CO), -114.64 (s, 2F, aryl F).
Crystal data for 3: C22H14Cl2F10N2O2Ti, M= 645.98 g mol-1, monoclinic, P21/n, a= 10.625(8), b= 15.630(9), c= 15.214(1), α=
90.00°, β= 98.05°, γ= 90.00°, V= 2502.0(7) Å3, T= 100 K, Z= 4, µ(Mo-K α)= 0.655 Å , Dc= 1.718 g*cm-3, 35051 reflections measured, 5010 independent (Rint= 0.0965), R1= 0.0505. wR2= 0.1315, 3762 independent observed reflections [I> 2σ(I)], 352 parameters.
Ethylene Polymerization. Ethylene polymerization was carried out under atmospheric pressure in toluene in a 500 mL glass reactor equipped with a mechanical stirrer. Toluene (200 mL) was introduced into the nitrogen-purged reactor and stirred vigorously (500 rpm). The toluene was kept at the desired polymerization temperature, and then the ethylene gas feed was started. After 15 min, polymerization was initiated by the addition of a toluene solution of MAO and then a toluene solution of complex into the reactor. The total volume of toluene in the reactor was 250 mL. After a prescribed time, ethanol (5 mL) was added to terminate the polymerization reaction, and the ethylene gas feed was stopped. The resulting mixture was added to the acidic methanol (1 mL of concentrated HCl in 500 mL of methanol).
The solid polyethylene was recovered by filtration, washed with methanol, and dried at 50 °C for 24 h in a vacuum oven.
RESULTS AND DISCUSSION
The ketoenamine 1 was obtained in good yield by acid-catalyzed condensation of the aniline with the diketone in toluene as a reaction medium removing the water formed by means of molecular sieves.
For the preparation of Zr complex 2, [Cl Zr(CH Ph) *THF] was utilized as a metal source. The reaction proceeds cleanly to afford a single isomer.
2 2 2
MCl2
2 N O F3C
F F
2: M= Ti 3: M= Zr
TiCl2
2 N O F3C
4
A crystal of complex 2 suitable for X-ray structure determination was grown from CH2Cl2 at - 30°C. In the solid state, complex 2 possesses an octahedral geometry around the zirconium center, analogous to Li’s titanium complex with β-enaminketonato ligand lacking 2,6-difluoro-substitution.5 The oxygen atoms (O-Zr-O angle, 162.57(7)°) are trans to each other, while the nitrogen atoms (N-Zr-N
Polymeric Materials: Science & Engineering2008, 98, 885
First publ. in: Polymeric Materials: Science and Engineering 98 (2008), pp. 885-886
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angle, 86.41(8)°) and the chloride ligands (Cl-Zr-Cl angle, 95.60(3)°) are situated in cis positions.
Figure 1. ORTEP drawing of the molecular structure of 2. The ellipsoids are shown with 50% probability. The H-atoms are not shown for clarity.
Due to the instability of the analogous titanium starting material [Cl Ti(CH Ph) ], [Cl Ti(NMe) ]2 2 2 2 2 was used as a metal source for the preparation of 3. The reaction of 1 with [Cl Ti(NMe) ]2 2 affords the monoamido-monochloro complex [(N^O)2TiCl(NMe2)], as revealed by additional studies. The compound was chlorinated without isolation with BCl3 to afford 3.
Single crystals of complex 3 suitable for x-ray diffraction were grown from toluene at room temperature. The structure of 3 (Figure 2) is similar to the Zr analog.
Figure 2. ORTEP drawing of the molecular structure of 3. The ellipsoids are shown with 50% probability. The H-atoms are not shown for clarity.
Preliminary studies of ethylene polymerization with both complexes using methylalumoxane (MAO) as a cocatalyst, were carried out at 25 °C under atmospheric pressure (Table 1). Both complexes were found to be active at 25 °C. In comparison to 4 lacking fluorine atoms adjacent to the imine nitrogen, complex 2 as well as the titanium analog displayed higher activities. In addition, the molecular weights of the polymers obtained are an order of magnitude larger than for the polymer obtained with 4. It’s also noteworthy that the polyethylenes produced with complex 2-3/MAO possess narrow polydispersities (Mw/Mn = 1.8 and 1.5, respectively). Thus, the preliminary studies indicate a suppression of chain transfer by the fluorine substitution.
Table 1. Polymerization of Ethylene by 2-3/MAO Systemsa
Complex
(µmol) t [min] TOFb Polymer [g]
Mnc (104 g mol-1)
Mw/ Mnc Tm
(°C)d 1 2 (1) 6 114 300 0.32 23.5 1.8 139 2 3 (1) 6 107 100 0.30 14.0 1.5 140
3 4 (3) 5 38
500 0.27 6.1 2.6 136
aReaction conditions: 1 atm ethylene pressure, toluene 250 mL, 2000 eq. MAO, 25°C (entry 3: toluene 100 mL, 2000 eq. MMAO).
bmol(ethylene)*mol(cat)-1*h-1, cDetermined by GPC vs linear polyethylene standards. dDetermined by DSC.
ACKNOWLEDGEMENT
We thank I. Göttker-Schnetmann for measuring the crystal structure of complex 2. Financial support by the DFG (Me1388/3-3) is gratefully acknowledged. S.M. is indebted to the Fonds der chemischen Industrie. We thank H. Brintzinger for stimulating discussions.
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